1. Field of the Invention
The present invention relates to an edge detecting device that detects an edge of a recording medium and an image forming apparatus including the edge detecting device.
2. Description of the Related Art
In an image forming apparatus of electrophotography, there has been a known technology of correcting main-scanning registration for each piece of recording media and a technology of calculating a skew amount from a detected edge of a recording medium by arranging an optical detecting unit such as a contact image sensor (hereinafter, referred to as a CIS) for detecting an edge of the recording medium (hereinafter, referred to as an “edge”) on a conveying path of the recording medium and by calculating a deviation amount of main-scanning registration from a detected edge position of the recording medium so as to adjust a transfer position relative to the recording medium or to adjust a position of the recording medium based on the transfer position.
An edge detection of a recording medium by a detecting unit of an optical system is performed by applying light emitted from a light source such as a light emitting diode (LED) included in the detecting unit onto the recording medium and by receiving reflected light reflected by the recording medium with a light receiving unit included in the detecting unit. For example, Japanese Patent Application Laid-open No. 2007-119135 discloses a configuration in which a CIS or the like having an LED as a light source is used as a detecting unit for detecting an edge of a recording medium and a light emission amount of the LED is adjusted so as to detect the edge of the recording medium.
In a conventional method for detecting an edge of a recording medium, detection light is applied onto the recording medium that is a detection target from a detecting unit of an optical system and reflected light reflected by the recording medium is received by a light receiving unit. Then, a processing unit processes in such a manner that the recording medium is determined to be present on a pixel that has received a greater amount of light than a threshold value and a recording medium is determined not to be present on a pixel that has not received a greater amount of light than the threshold value. Furthermore, a boundary between these pixels is detected to be the edge of the recording medium. Therefore, reflection by the recording medium may be insufficient depending on the wavelength of light emitted from a light source of the detecting unit to be used and conditions originating from the recording medium, such as the material properties or the color of the recording medium. In such a case, the detecting unit may receive the reflected light with the amount that does not exceed the threshold value even on a portion on which the recording medium is present, thereby making the determination on whether or not the recording medium ambiguous. Therefore, it is difficult to detect an edge of the recording medium with high accuracy.
Japanese Patent Application Laid-open No. 2007-119135 discloses a technology in which the CIS or the like is arranged for detecting an edge of a recording medium and a light emission amount of the LED is adjusted so as to detect the edge of the recording medium properly. However, there are problems which cannot be solved easily. For example, choice on the light emission wavelength of a light source of a detecting unit (CIS) to be used and the reflectance property of the recording medium may make it difficult to detect an edge of a recording medium. Furthermore, detection of an edge of a recording medium having low reflectance requires the light emission time of the light source to be longer and the light emission intensity of the light source to be higher, thereby to shorten the lifetime of the light source and to increase detection time needed for detecting the edge of the recording medium.
Thus, there is a need to provide an edge detecting device that can detect edges of various types of recording media accurately in a short period of time independent of the emission wavelength of the light emitted from a light source or reflectance properties of the recording media.
There is also a need to provide an image forming apparatus that has high paper handling capacity and can realize a preferable image by achieving high accuracy in detecting edges of recording media in a short period of time even when various types of recording media are used.